Analysis of resting noise characteristics of three EIT systems in order to compare suitability for time difference imaging with scalp electrodes during epilepsy

Abstract

We have investigated the noise in three systems suitable for imaging epileptic seizures, the UCH Mark1b, UCH Mark2.5 and KHU Mark1 16 channel systems, at applied frequencies in three steps from 1 to 100 kHz, by varying load impedance, single terminal or multiplexed measurements, and in test objects of increasing complexity from a resistor to a saline filled tank and scalp electrodes in a human volunteer. The noise was white, and increased from about 0.03% on the resistor to 0.08% on the human; it increased with load but was independent of use of the multiplexer. The KHU Mark1 system delivered the best performance with noise spectra of about 0.02%, which could be further reduced by averaging to a level where reliable imaging of changes of about 0.1% estimated during epileptic seizures appears plausible. This appears to be because the KHU Mark1 records in parallel and employs screened triaxial cables, unlike the other two systems. Work in progress is to evaluate the KHU Mark1 with longer 5m leads which are needed for clinical studies in ambulant patients and optimisation of current levels in the UCH systems. Following this, the two best systems will be evaluated in clinical studies in patients during epileptic seizures. © Springer-Verlag 2007.